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Astronomy Calculations


Content:

Minimize vignetting calculator:


In an optical system you want to minimize the vignetting, you don't want to loose any light. This is not about the normal circular vignetting. With this calculator you can get a feeling of the minimum diameter of your filter and the minimum distance from the prism in an off-axis adapter to the optical axis. Anything that protrude into the light cone between the lens to the sensor will cause vignetting.

Here I try to sort it out:

Minimize vignetting calculator

Overview:

Minimize vignetting calculater

Type in your data

It start to calculate as soon you change or write new figures in the white or dark red boxes. Do not exceed the maximum number of characters, delete characters if necessary.

Note:
You use the information on your own risk! There can always be a mistake in my equations behind the calculations, check that the result is correct. Let me know if you find something wrong and I try to correct it. Some calculations are very simple done and not correct in the small details.


Follow the steps:

Note:
You can always override the limits by typing in the figures direct in the window. In some cases you maybe get strange results.

1, Opening diameter of lens or mirror ?

Find the diameter of your telescope's opening, the free opening i.e. lens or mirror diameter.

Type in the diameter:

Free opening diameter
(0 to 999 mm)

2, The focal length ?

Type in the focal length:

focal length     f-number
Focal length   = f-number
(100 to 4000 mm)     ratio

3, Reducer / Barlow ?

Type in the magnification, 1= none:

magnification     new focal length
Magnification   = new focal length
(0.20 to 5.00 mag)     new f-number
      new f-number
This is very simplified.     ratio

4, length and height of sensor ?

Type in the dimension of the sensor:

Sensor length Sensor height   Sensor diagonal
= Sensor diagonal
(4 to 70 mm) (3 to 60 mm)   mm

The length must always be longer than the height or equal.

5, filter data ?

Type in the distance from filter to sensor:

Distance     Minimum filter diameter
circular filter = Minimum filter diameter
(4 to 150 mm)     diameter mm
  rectangular filter Minimum filter length Minimum filter height
    length mm height mm

This is the free opening of the filter, add at least 2 mm to the minimum diameter. The most common rectangular filter is the 50 x 50 mm. Too small and it will cause vignetting, too big and it will empty your wallet. Note: Many strong reducers have very small output lens diameters and can't take advantage of a correct designed filter diameter, no need to have a bigger diameter on the filter than the output lens of the reducer.

The filter must be centered exact around the optical axis, the sensor too, otherwise you need a bigger diameter filter. If you look down into the telescope you can normally see if there is something blocking the light ray, have the shutter open. Move the eye from side to side of the lens (I haven't tried this on a mirror telescope), even at the edges of the lens you shall see the whole sensor at bottom of the telescope.

6, off-axis adapter data ?

Type in the distance from center of prism to sensor:

Distance     minimum distance
from optical axis
= Minimum distance from
prism to optical axis
(4 to 150 mm)     mm

This is the case when the prism is directed to the center of the long side of the sensor. Minimum distance from the bottom of the prism to optical axis, add at least 2 mm to the minimum distance. This is normally a trail and error process. You want it to be as close as possible but not cause any vignetting. See my tutorial how to setup an off-axis adapter.

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